Most Java applications require a persistent class representing felines.

package eg;
import java.util.Set;
import java.util.Date;

public class Cat {
    private Long id; // identifier
    private String name;
    private Date birthdate;
    private Cat mate;
    private Set kittens
    private Color color;
    private char sex;
    private float weight;

    private void setId(Long id) {
        this.id=id;
    }
    public Long getId() {
        return id;
    }

    void setName(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }

    void setMate(Cat mate) {
        this.mate = mate;
    }
    public Cat getMate() {
        return mate;
    }

    void setBirthdate(Date date) {
        birthdate = date;
    }
    public Date getBirthdate() {
        return birthdate;
    }
    void setWeight(float weight) {
        this.weight = weight;
    }
    public float getWeight() {
        return weight;
    }

    public Color getColor() {
        return color;
    }
    void setColor(Color color) {
        this.color = color;
    }
    void setKittens(Set kittens) {
        this.kittens = kittens;
    }
    public Set getKittens() {
        return kittens;
    }
    // addKitten not needed by Hibernate
    public void addKitten(Cat kitten) {
        kittens.add(kitten);
    }
    void setSex(char sex) {
        this.sex=sex;
    }
    public char getSex() {
        return sex;
    }
}

There are four main rules to follow here:

A subclass must also observe the first and second rules. It inherits its identifier property from Cat.

package eg;

public class DomesticCat extends Cat {
        private String name;

        public String getName() {
                return name;
        }
        protected void setName(String name) {
                this.name=name;
        }
}

You have to override the equals() and hashCode() methods if you intend to mix objects of persistent classes (e.g. in a Set).

This only applies if these objects are loaded in two different Sessions, as Hibernate only guarantees JVM identity ( a == b , the default implementation of equals()) inside a single Session!

Even if both objecs a and b are the same database row (they have the same primary key value as their identifier), we can't guarantee that they are the same Java instance outside of a particular Session context.

The most obvious way is to implement equals()/hashCode() by comparing the identifier value of both objects. If the value is the same, both must be the same database row, they are therefore equal (if both are added to a Set, we will only have one element in the Set). Unfortunately, we can't use that approach. Hibernate will only assign identifier values to objects that are persistent, a newly created instance will not have any identifier value! We recommend implementing equals() and hashCode() using Business key equality.

Business key equality means that the equals() method compares only the properties that form the business key, a key that would identify our instance in the real world (a natural candidate key):

public class Cat {

    ...
    public boolean equals(Object other) {
        if (this == other) return true;
        if (!(other instanceof Cat)) return false;

        final Cat cat = (Cat) other;

        if (!getName().equals(cat.getName())) return false;
        if (!getBirthday().equals(cat.getBirthday())) return false;

        return true;
    }

    public int hashCode() {
        int result;
        result = getName().hashCode();
        result = 29 * result + getBirthday().hashCode();
        return result;
    }

}

Keep in mind that our candidate key (in this case a composite of name and birthday) has to be only valid for a particular comparison operation (maybe even only in a single use case). We don't need the stability criteria we usually apply to a real primary key!

Optionally, a persistent class might implement the interface Lifecycle which provides some callbacks that allow the persistent object to perform necessary initialization/cleanup after save or load and before deletion or update.

The Hibernate Interceptor offers a less intrusive alternative, however.

public interface Lifecycle {
        public boolean onSave(Session s) throws CallbackException;   (1)
        public boolean onUpdate(Session s) throws CallbackException; (2)
        public boolean onDelete(Session s) throws CallbackException; (3)
        public void onLoad(Session s, Serializable id);              (4)
}

onSave(), onDelete() and onUpdate() may be used to cascade saves and deletions of dependent objects. This is an alternative to declaring cascaded operations in the mapping file. onLoad() may be used to initialize transient properties of the object from its persistent state. It may not be used to load dependent objects since the Session interface may not be invoked from inside this method. A further intended usage of onLoad(), onSave() and onUpdate() is to store a reference to the current Session for later use.

Note that onUpdate() is not called every time the object's persistent state is updated. It is called only when a transient object is passed to Session.update().

If onSave(), onUpdate() or onDelete() return true, the operation is silently vetoed. If a CallbackException is thrown, the operation is vetoed and the exception is passed back to the application.

Note that onSave() is called after an identifier is assigned to the object, except when native key generation is used.

If the persistent class needs to check invariants before its state is persisted, it may implement the following interface:

public interface Validatable {
        public void validate() throws ValidationFailure;
}

The object should throw a ValidationFailure if an invariant was violated. An instance of Validatable should not change its state from inside validate().

Unlike the callback methods of the Lifecycle interface, validate() might be called at unpredictable times. The application should not rely upon calls to validate() for business functionality.

In the next chapter we will show how Hibernate mappings may be expressed using a simple, readable XML format. Many Hibernate users prefer to embed mapping information directly in sourcecode using XDoclet @hibernate.tags. We will not cover this approach in this document, since strictly it is considered part of XDoclet. However, we include the following example of the Cat class with XDoclet mappings.

package eg;
import java.util.Set;
import java.util.Date;

/**
 * @hibernate.class
 *  table="CATS"
 */
public class Cat {
    private Long id; // identifier
    private Date birthdate;
    private Cat mate;
    private Set kittens
    private Color color;
    private char sex;
    private float weight;

    /**
     * @hibernate.id
     *  generator-class="native"
     *  column="CAT_ID"
     */
    public Long getId() {
        return id;
    }
    private void setId(Long id) {
        this.id=id;
    }

    /**
     * @hibernate.many-to-one
     *  column="MATE_ID"
     */
    public Cat getMate() {
        return mate;
    }
    void setMate(Cat mate) {
        this.mate = mate;
    }

    /**
     * @hibernate.property
     *  column="BIRTH_DATE"
     */
    public Date getBirthdate() {
        return birthdate;
    }
    void setBirthdate(Date date) {
        birthdate = date;
    }
    /**
     * @hibernate.property
     *  column="WEIGHT"
     */
    public float getWeight() {
        return weight;
    }
    void setWeight(float weight) {
        this.weight = weight;
    }

    /**
     * @hibernate.property
     *  column="COLOR"
     *  not-null="true"
     */
    public Color getColor() {
        return color;
    }
    void setColor(Color color) {
        this.color = color;
    }
    /**
     * @hibernate.set
     *  lazy="true"
     *  order-by="BIRTH_DATE"
     * @hibernate.collection-key
     *  column="PARENT_ID"
     * @hibernate.collection-one-to-many
     */
    public Set getKittens() {
        return kittens;
    }
    void setKittens(Set kittens) {
        this.kittens = kittens;
    }
    // addKitten not needed by Hibernate
    public void addKitten(Cat kitten) {
        kittens.add(kitten);
    }

    /**
     * @hibernate.property
     *  column="SEX"
     *  not-null="true"
     *  update="false"
     */
    public char getSex() {
        return sex;
    }
    void setSex(char sex) {
        this.sex=sex;
    }
}